There is a desire in the industry for higher circuit density in microelectronic devices which are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. The use of shorter wavelength radiation (e.g. deep UV e.g. 190 to 315 nm) than the currently employed mid-UV spectral range (e.g. 350 nm to 450 nm) offers the potential for higher resolution. However, with deep UV radiation, fewer photons are transferred for the same energy dose and higher exposure doses are required to achieve the same desired photochemical response. Further, current lithographic tools have greatly attenuated output in the deep UV spectral region.
In order to improve sensitivity, several acid catalyzed chemically amplified resist compositions have been developed such as those disclosed in U.S. Pat. No. 4,491,628 (Jan. 1, 1985) and Nalamasu et al., "An Overview of Resist Processing for Deep-UV Lithography", J. Photopolym Sci. Technol. 4, 299 (1991). The resist compositions generally comprise a photosensitive acid generator and an acid sensitive polymer. The polymer has acid sensitive side chain (pendant) groups which are bonded to the polymer backbone and are reactive towards a proton. Upon imagewise exposure to radiation, the photoacid generator produces a proton. The resist film is heated and, the proton causes catalytic cleavage of the pendant group from the polymer backbone. The proton is not consumed in the cleavage reaction and catalyzes additional cleavage reactions thereby chemically amplifying the photochemical response of the resist. The cleaved polymer is soluble in polar developers such as alcohol and aqueous base while the unexposed polymer is soluble in nonpolar organic solvents such as anisole. Thus the resist can produce positive or negative images of the mask depending of the selection of the developer solvent.
Although chemically amplified resist compositions generally have suitable lithographic sensitivity, in certain applications, their performance can be improved by (i) increasing their thermal stability in terms of thermal decomposition and plastic flow and (ii) increasing their stability in the presence of airborne chemical contaminants. For example, in some semiconductor manufacturing processes, post image development temperatures (e.g. etching, implantation etc.) can reach 200.degree. C. Brunsvold et al., U.S. Pat. Nos. 4,939,070 (issued Jul. 3, 1990) and 4,931,379 (issued Jun. 5, 1990) disclose chemically amplified, acid sensitive resist compositions having increased thermal stability in the post-image development stage. Brunsvold's resist compositions form a hydrogen bonding network after cleavage of the acid sensitive side chain group to increase the thermal stability of the polymer. Brunsvold avoids hydrogen bonding moieties prior to the cleavage reaction because such hydrogen bonding is known to unacceptably thermally destabilize the acid sensitive side chain. Although Brunsvold resists have suitable thermal stability, they also have lower sensitivity and therefore are unsuitable in certain applications.
With respect to chemical contamination, MacDonald et al. SPIE 1466 2, (1991) reported that due to the catalytic nature of the imaging mechanisms, chemically amplified resist systems are sensitive toward minute amounts of airborne chemical contaminants such as basic organic substances. These substances degrade the resulting developed image in the film and cause a loss of the linewidth control of the developed image. This problem is exaggerated in a manufacturing process where there is an extended and variable period of time between applying the film to the substrate and development of the image. In order to protect the resist from such airborne contaminants, the air surrounding the coated film is carefully filtered to remove such substances. Alternatively, the resist film is overcoated with a protective polymer layer. However, these are cumbersome processes.
Therefore, there still is a need in the art for an acid sensitive, chemically amplified photoresist composition having high thermal stability and stability in the presence of airborne chemical contaminants for use in semiconductor manufacturing.
It is therefore an object of the present invention to provide an improved acid sensitive, chemically amplified photoresist composition.
Other objects and advantages will become apparent from the following disclosure.